Sex differences in the brain may underlie risk for neurological disorders. Arginine vasopressin (AVP) is implicated in several behavioral disorders and acts in the brain to influence social behaviors in rats. Male rats express more AVP than females within several brain regions, and this is largely organized by testosterone and its metabolites during early postnatal development. However, the mechanism by which these hormones establish male-typical AVP remains unclear. I hypothesized that hormones organize sex differences in AVP through epigenetic modifications. First, I examined whether methyl-CpG-binding protein 2 (MeCP2) plays a role in the organization of AVP expression within the amygdala by infusing MeCP2 siRNA into the amygdala during the first three days of postnatal life. I found that a transient reduction in MeCP2 decreases AVP, galanin, and AR expression within the male amygdala. Only the effect on AVP lasts into adulthood, highlighting a unique function of MeCP2 in sexual differentiation. I then examined whether there are sex differences in DNA methylation within the Avp promoter region that correspond to the sex difference in AVP expression, and whether these sex differences are organized by hormones during development. I found that females had higher levels of methylation than males at two CpG sites within the Avp promoter region, and estradiol decreased methylation at one of these sites. While the importance of AVP regulation by testosterone is well known, data also suggest that progesterone acts upon progestin receptors (PRs) to suppress AVP and AVP-dependent social recognition. I examined the role of PRs in the organization of AVP-dependent behaviors and AVP expression during early postnatal life. Blocking PRs improved social recognition in juvenile animals and increased juvenile social play in females to male-typical levels. AVP mRNA was increased within the female, but not the male amygdala. Furthermore, females had higher methylation at two CpG sites within the Avp promoter region than males, and blocking PRs decreased methylation in females. These data suggest that testosterone organizes AVP in part through epigenetic modifications. Furthermore, PRs also play a role in its organization. This research contributes to our understanding of the epigenetic regulation of sex differences.